Toss the tomatoes, cucumbers, and lemon juice in a bowl with 1/2 teaspoons of the salt. Set aside.

Put the couscous into a large bowl. Bring the water, lemon zest, oil, the remaining salt, cayenne, and herb stems to a boil over high heat. Pour the liquid over the couscous, stir to separate any clumps. Cover with a lid, plate, or plastic wrap, set aside for 5 minutes. Remove the zest and herb stems and fluff with a fork.